Transcript for President Obama Responds to James Foley's Execution, Promising to be "Vigilant and Relentless."

And now, the question continues. What is the U.S. Going to do. Isis is now estimated to be more than 10,000 strong and growing. Have taken over a third of Iraq and parts of Syria. Jonathan Karl breaks down president Obama's options. Reporter: He calls out president Obama by name. The president answering with resolve. We will be vigilant and relentless. When people harm Americans anywhere, we will work to see that justice is done. Reporter: The U.S. Has conducted at least 13 more air strikes since the video was released yesterday. He could release more weapons to the front line, or send special forces to advice. He could launch air strikes for the first time on Isis targets in Syria. Now, the president faces a new fight in Iraq that could dominate the rest of his presidency. And now, ABC's Pierre Thomas, who has been talk to his expert sources today about the number of westerners joining the terrorists. Well, as Brian pointed out, the executioner seemed to be british. This is 19-year-old Shannon Conley of Denver. Looks like an all-american girl next door, but authorities say she was hell bent on joining Isis. The fear tonight, they come back to America to strike. There's an effort to track them, some back here at home under various levels of investigation and surveillance. It has everyone on edge there. Thank you, Pierre Thomas. We'll have every breaking development on abcnews.com. We move on to the monsoon conditions slamming the west in

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